Various abbreviations that may appear in the specification and/or in the drawing figures are defined as follows:    3GPP third generation partnership project    UTRAN universal terrestrial radio access network    Node B base station    UE user equipment    HO handover    EUTRAN evolved UTRAN    eNB EUTRAN Node B (evolved Node B)    MME mobility management entity    SAE system architecture evolution    RLC radio link control    LTE long term evolution    OFDMA orthogonal frequency division multiple access    SC-FDMA single carrier, frequency division multiple access    UL uplink    DL downlink    O&M operation and maintenance    RAT radio access technology
A communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA) is currently under discussion within the 3GPP. As currently specified the DL access technique will be OFDMA, and the UL access technique will be SC-FDMA.
One specification of interest to the ensuing discussion of the background, and the exemplary embodiments of this invention, is 3GPP TS 36.300, V8.5.0 (2008-05), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 8), incorporated by reference herein in its entirety.
FIG. 2 reproduces FIG. 4 from 3GPP TS 36.300, V8.5.0 (2008-05) and shows various eNBs, MME/SAE gateways, and the interfaces (S1, X2) between these components in the E-UTRAN (LTE) context.
For LTE a scattered eNB deployment is considered as being an essential pre-requisite. Due to the fact that there can be different HO algorithms of different vendors of eNBs (and UEs), the potential for occurrence of the ping pong effect is significant.
Briefly, the ping pong effect can occur if a UE has very similar radio conditions towards two eNBs, thus eNB_A may handover the UE to eNB_B, which then results in a handover in the opposite direction (from eNB_B to eNB_A). Handover decisions are typically based on certain fixed, often vendor specific, differences in cell signal levels, which are potentially different in base stations from different vendors, in combination with momentary variations of either of the links. As a result, at any given time one of the base stations may appear preferable to the other and fulfill the HO requirement. As can be appreciated, this results in additional and unnecessary signaling load on the wireless and wired interfaces, as well as S1 re-direction and UE data forwarding via the X2 interface, even though both eNBs would be capable of serving that particular UE.
Although the ping pong problem is known in current wireless networks (non-LTE networks), it is believed that scattered Node B deployments are intentionally avoided by the network operators. In a single vendor Node B area all Node Bs are based on and operate with the same HO-related algorithms. This can mitigate the ping pong problem to a large extent (which may be handled simply by the use of timers), since the Node Bs may be assumed to operate using the same algorithm and HO decision criteria. In the idle mode cell individual (fixed) offsets may be defined to minimize the ping pong occasions due to link quality fluctuations. Nevertheless, ping ponging still occurs in certain scenarios, resulting in a high signaling load.